The present invention relates to basic bonded fluxes for submerged arc welding in which slags are easily removed at a narrow groove and aims to improve the removability of slags when a narrow groove in steel plates having a particularly large thickness is deposited by submerged arc welding process through one pass-one layer technique and to stabilize a molten metal flow movement in the weld puddle to prevent the formation of the welding defect and increase the qualities of the weld joint.
As a highly reliable welding process for steel materials having a very large thickness, such as off-shore structures, nuclear reactors, pressure vessels and the like, submerged are welding has been generally advantageously used and a prior embodiment of a weld joint configuration and welding sequence is shown in FIG. 1 and in general, a width W of bottom of a groove is about more than 25 mm and the bead is deposited in multi-layers with two or more passes.
The reason why such a broad groove is adopted is that it is difficult to remove slags formed at every pass in a narrow groove where the distance of the groove is small and the removability of the slags is noticeably deteriorated to cause problems in the welding workability and further the welding defects, such as slag inclusion and lack of fusion are readily caused.
For a conventional broad groove, a number of welding pass is noticeably increased particularly in heavy plates having a larger thickness than 50 mm and therefore a large amount of welding materials and a long arc time are required and the efficiency becomes very poor.
In general, the welding fluxes are classified into acidic fluxes and basic fluxes.
When acidic fluxes are used, the oxygen content of weld metal is very high and therefore it is difficult to obtain excellent toughness and since a large amount of diffusible hydrogen is contained in the weld metal, hydrogen cracks are induced in welded joints having a heavy thickness.
On the other hand, oxygen and diffusible hydrogen levels in the weld metal in the case of basic fluxes may be lower than those in the case of acidic fluxes and the basic fluxes are effective for the improvement of the joint qualities. However, a basic weld slag has generally crystalline structure which is strong and is hardly crushed and the operation for removing the weld slag is very difficult.
The applicant has already proposed in Japanese Patent Application No. 164,692/79 that the removability of the slag is effectively improved by selecting the composition of a flux which mainly forms crystals of periclase (MgO) and barium mica (BaMg.sub.3 Al.sub.2 Si.sub.2 O.sub.10 F.sub.2) in the slag after welding.
When a groove having a narrow gap is subjected to the submerged arc welding through one pass-one layer technique as shown in FIG. 2 by using the above described flux, it has been necessary to cause a moderate arc blow by severely controlling a height of a flux dispersed to 45.+-.5 mm in order to form a concaved weld bead surface and the control of the welding conditions have become troublesome. Thus, the inventors have in detail observed and checked the phenomenon of a molten pool by using a X-ray image amplifier apparatus concerning this cause and as the result it has been found that when welding defects, such as inadequate bead surface, undercut and slag inclusion and the like are formed, the flow of molten metal in the weld puddle is very unstable and that since the structure of the weld slag is crystalline, the slag wedged in the undercut is hardly crushed to facilitate its removal.